'so the whole design was adopted for 300 km'.
Doesn't look to me that he is saying more range is possible. Idk where you get that it has more range or 500 km part. And if it did we wouldn't be making LRCM with only a 100 km range boost now would we?
He clearly stated that we settled for design that will give us 300 km ( in fact officially stated range is 290 km for obvious reasons ) so as to honour Russia's commitment to MTCR . He also stated that the range can be increased if required quite clearly . How much range can be extended ? that he did not answer and can't answer. But there are great many experts who believe that it can be extended to 500 km ...
No we are not making Long Range Supersonic Cruise Missile with only 100 km additional range i.e 600 km range .The diagram that I posted was taken from DRDO ppt mant years back when Brahmos was still making progress .
Frankly speaking asides the sole DRDO presentation we have no additional info about LFRJ project in public domain . so we can only speculate . In fact that's why people have confused between Turbofan based Nibhay ( often called as -LRCM -Long range subsonic Cruise Missile ) with Ramjet based LFRJ (LRCM - Long Range Supersonic Cruise Missile ) as in the article OP has posted .
It is quoted that LFRJ will produce missile with range of 1000 km or so ...but all this is a speculation !
at the same time it is inconceivable that when DRDO and ISRO are trying to extend exploits of ramjet technology with their HSTDV and RLV TD projects ...the ramjet technology won't be used to develope next generation long range cruise missiles ...